Looking for gravitational waves in a sea of noise
From a mountain top reaching 5080 meters above sea level, situated in the driest desert in the world, some of the world’s most sensitive arrays of “miniature TV antennas” have spent the last 30 months gazing at the sky, looking for tiny wrinkles in the fabric of space itself: Wrinkles that would reveal what the universe looked like when it was only 10-34 seconds old; wrinkles with a relative amplitude of perhaps no more than a few parts in a billion; and wrinkles that would qualify their discoverer for a Nobel prize.
Heavy objects orbiting each other produce ravitational waves. Such waves were also produced shortly after the Big Bang, and CMB experiments, like QUIET, are today trying to detect these waves. Photo: NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory)
This experiment is called the “Q/U Imaging ExperimenT” (“QUIET” for short), and is one of several experiments competing to become the first to detect the tiny signal.
Right in the middle of this race are the University of Oslo and NOTUR, whose computational expertise and resources have enabled Norwegian scientists to take on leading roles in the experiment.
Albert Einstein, gravity waves and the biggest bang of them all
The background for the QUIET experiment begins almost 100 years ago, on November 25th 1915, to be precise. On this date, Albert Einstein submitted a paper called “The field equations of gravitation” to the Prussian science academy, which provides a fundamentally new understanding of gravity.
While Newton believed that gravity was a somewhat mysterious force that pulled bodies together through large distances, Einstein claimed that gravity was nothing but the manifestation of a curved spacetime, and any body simply attempts to move in the straightest possible line within this curved space.
This idea can be illustrated by a pilot flying around the world: Locally, the pilot always attempts to fly in the straightest possible line, and yet, after some 36 hours he comes back to the point where he started his flight, because the Earth is spherical.
As with any great scientific theory, this idea could be tested by observations. The first opportunity to do so came in 1919, when British astronomer Lord Eddington measured the bending of light around the Sun during a total solar eclipse: According to Einstein’s theory the amount of bending would be twice as large as predicted by Newton’s theory. In a spectacular media display, Eddington confirmed Einstein’s predictions, and Einstein rose to instant world fame.
Einstein’s theory made many other predictions as well. For QUIET, two are of particular interest. First, according to Einstein the gravitational field set up by a massive body should behave in a similar way as the electromagnetic field: Disturbances in this field should move at the speed of light, just like photons, forming what is called gravitational waves.
Such waves arise whenever strong gravitational sources accelerates, say, when two black holes orbit each other. However, the physical magnitude of such variations is typically very small indeed: The waves produced by the Earth orbiting the Sun correspond to a stretching of space by only one part in 1026! For this reason, no gravitational waves have been directly observed as of 2011.
The second important prediction is that the universe as a whole expands, as was later observed directly by Edwin Hubble in 1929. This effect can be compared to the surface of an inflating balloon; as the balloon inflates its surface area becomes larger and larger, and the distance between any two points on the surface increases.
The same happens in the universe; all galaxies move away from each other. And this has a very interesting and simple consequence: If the universe expands today, then it must have been smaller in the past. And that means that the density of the universe must have been higher in the past. And when the density increases, so does the temperature.
Taking this idea to its extreme, Einstein’s theory therefore makes the following simple prediction: The universe started as an extremely hot and dense gas, in which only the very simplest elementary particles could exist; anything heavier than photons, protons and electrons would be instantly destroyed by the extreme heat. This idea is today known as “the Big Bang”.
The cosmic microwave background and two Nobel prizes
Until 1965, the Big Bang theory was mostly a theoretical speculation, based on mathematics rather than hard observations. However, this all changed when two scientists at Bell Laboratories, Arnos Penzias and Robert Wilson, accidentally made the discovery of a lifetime: While they were trying to make a picture of the Milky Way using a new radio antenna, they found an unexpected “noise term” in their observations of 3 degrees Kelvin.
And this signal had one very peculiar property: It was equally strong in all directions on the sky. No known signal behaved like this. Except the residual radiation from the Big Bang, now cooled from an original temperature of 3000K to 3K.
Penzias and Wilson had discovered the echo from the Big Bang by accident, today known as the “cosmic microwave background”, or CMB for short. For this, they received the Nobel Prize in physics in 1978.
Shortly after the first detection of the CMB by Penzias and Wilson, it was realized that the background should in fact not be perfectly uniform, but rather have tiny fluctuations around the mean. The reason is simply that we see structure in the universe around us today; there are galaxies, solar systems, and even us. If the universe had been perfectly smooth early on, it would remain so to the very end.
Further, if it was possible to measure these tiny fluctuations, cosmologist (that is, physicists which study the entire universe) found that it would be possible to extract an immense range of cosmological information from them. Everything from the age and contents of the universe to the evolution and destiny would be constrained.
Needless to say, an intense flurry of activity started, as physicists started to build more and more sensitive instruments to make detailed measurements. The breakthrough came with the NASA satellite called “the Cosmic Background Explorer” (COBE), which found the first signs of CMB fluctuations in 1992. These measurements showed that there were indeed small perturbations in the CMB, corresponding to temperature variations on the sky of ~10 muK -- just as the theorists had predicted almost forty years earlier.
While the discovery of Penzias and Wilson can be said to change cosmology from a field of speculation into a proper branch of physics, the COBE discovery opened a new window of the early universe, which eventually transformed cosmology into a highprecision science. For this, the leaders of the COBE mission, John Mather (NASA) and George Smoot (Berkeley), received the Nobel prize in physics in 2006.
Tiny fluctuations in the temperature of the CMB, as measured by the WMAP satelite. The red and blue spots in this map correspond to places in the universe with lower and higher density than the average. The high-density spots later evolved into galaxy structures. Photo: NASA
Since then, many experiments have improved greatly on the observations made by COBE. Two particularly important experiments are WMAP, a second NASA-funded satellite which operated between 2001 and 2010, and Planck, an ESA-funded satellite currently taking data. The University of Oslo is a member of Planck, and scientists at the Institute of Theoretical Astrophysics (ITA) are just these days analyzing the data that are sent down from this satellite.
Once Planck is finished with its job, it will provide a map with sufficient resolution and signal-to-noise that it should never be necessary to measure it again.
Inflation and the birth of all galaxies
However, just as one chapter of CMB cosmology is about to end, another is just starting. To explain the background for this, it is necessary to go back to 1981, when theorists were asking themselves one important question: Why do we observe the same temperature in two opposite directions on the sky the same? Since the photons we observe at the Earth today have been travelling towards us ever since the beginning of time, for more than 13 billion years, it would take 26 billion years for one of those photons to reach the birth place of the second photons. And since the universe is indeed only 13 billion years old, there is no time for this to have happened.
So what was going on? To explain this paradox, theorists proposed something very strange, today known as cosmic inflation: When the universe was only some 10-34 seconds old, it underwent a very short period of extremely rapid expansion, during which it increased in size by a factor of 1026! For comparison, that is about the same as inflating a balloon to the size of the entire observable universe -- in only 10-35 seconds!
As crazy as this idea sounds, it did have many attractive features. First, it could explain many of the paradoxes that plagued theoretical cosmology at the time; the smoothness of the universe is one example, the flatness of the universe is another.
Second, it also provided a physical mechanism for how the very first structures in the universe were generated: According to the inflationary theory, everything we see around us started simply as quantum fluctuations during that epoch of rapid expansion 10-34 seconds after the Big Bang. Third, and most importantly, the theory made concrete predictions that could be tested observationally.
The predictions made by inflation can be divided into two classes. First there are the circumstantial pieces of evidence: If the inflationary scenario is right, the fluctuations we observe in the universe should be what is called isotropic (ie., look statistically similar in all directions on the sky), scale-invariant (ie., there should be no preferred length scale in the universe), and Gaussian (ie., noise-like). The followup experiment to COBE, WMAP, has verified all these predictions, and they appear to be OK.
Unfortunately, these were all predicted by cosmologists *before* inflation came along in 1981, and they can therefore not be taken as evidence of inflation. However, the second class of evidence is something that is unique for inflation: If inflation is correct, there should be a background of gravity waves permeating the universe, quantum mechanically created during the violent period of rapid expansion.
This signature is almost impossible to generate by any other mechanism. If ever detected, this signature will therefore provide direct evidence for inflation, and thereby the creation of everything we see around us today. And as if that was not enough, this detection will also be the first direct evidence of gravity waves, predicted by Einstein almost 100 years ago.
It should come as no surprise that the discoverer of such fundamental physics will surely qualify for a new Nobel prize. And similarly, it should come as no surprise that scientists all over the world are currently looking very hard for these gravity waves. And QUIET is one of the main players in this game.
Gravity waves, CMB polarization and B-modes
The most promising route toward detecting these primordial gravity waves is once again provided by the CMB fluctuations, but this time in a more subtle way than for the simple density fluctuations. The idea is the following: When a gravity wave moves through space, the space expands in one direction and compresses in the other. And when space itself expands or contracts, so does the photons within it. This, in turn, leads to a *polarized* CMB signal: The observed signal from a single point on the sky is hotter in one polarization component than the other.
Unfortunately, primordial gravity waves is not the only physical mechanism that generates polarization, as the same under-and overdensities we observe in the unpolarized CMB also generate a pattern of polarization fluctuations on their own, even in the absence of gravity waves. However, the gravity waves signal has one particular feature that does distinguish it from other signal: It has a so-called divergence-free part called “B-modes”, behaving mathematically similar to a magnetic field. This component is the unique feature of early universe gravity waves that everybody is searching for.
Above we said that the magnitude of the temperature fluctuations were small, having an amplitude of only some ~10 muK relative to an absolute background of 3K. However, the B-modes are smaller yet, with an expected amplitude of perhaps 0.1 muK or even less. In other words, CMB polarization experiments are looking for fluctuations that are seven orders of magnitude smaller than the mean on which they sit! For comparison, that is like measuring the height of a person to an accuracy of one thousandth of a millimeter!
The QUIET experiment
To produce such high sensitivity and clean pictures of the CMB sky, it is necessary to build extremely sensitive detectors -- and many of them. This poses many serious technical challenges, two of which are size and cost. Ten years ago, the typical CMB detector (called radiometer) was built by hand, and was about 30 cm end-to-end. The cost of a single device was therefore high, running at about $40,000, and it took a long time to calibrate and test it. Further, it was physically impossible to put many of these into a single telescope, since they were so large.
However, in 2003 a major breakthrough was made at the Jet Propulsion Laboratory (JPL/NASA), where two scientists, Drs. Todd Gaier and Michael Seiffert, managed to design a fully functional radiometer on a 3-by-3 cm chip (MMIC), using similar techniques as those used for producing computer processors. This solved both of the two mentioned problems: First, the size became small enough to allow for a large array of the devices in a single telescope, and the cost of a single detector dropped to about $500.
Quickly realizing the potential massive scientific importance of this breakthrough for CMB cosmology, scientists from many institutions and countries organized themselves to establish what is today known as the QUIET experiment. The goal of this experiment was to build and field a telescope based on the new detector technology, attempting to measure the much sought-after gravity wave signal from inflation. Today, this has grown into a collaboration of 14 top institutions around the world (Caltech, Chicago, Columbia, Fermilab, JPL, KEK (Japan), Manchester, Max-Planck Institut (Germany), Miami, Michigan, Oslo, Oxford, Princeton and Stanford), and about 50 scientists are working on it.
While having excellent detectors is crucial for searching for the CMB polarization signal, this is not sufficient by itself. One problem is the Earth’s atmosphere. Water vapour absorbs radio waves, and at some frequencies the atmosphere is
completely opaque. It is therefore better to observe through less atmosphere, and in a dry environment. For this reason, QUIET is located at the Chajnantor plateau, 5080 meters above sea level, in the Atacama desert in Chile, the driest desert in the world. The only other site with competitive qualities in the world is the South Pole, which some CMB experiments do use for their observations. However, due to rather obvious infrastructure advantages, the Chile site was preferred for QUIET.
The QUIET telescope, located 5080 m above sea level in the Atacama desert in Chile. Photo: QUIET
Equally important is it to suppress any form of systematic effects. Again, the gravity wave signal one is looking for has an amplitude of about 0.1 muK; for comparison, the surrounding environment temperature is about room temperature, 300K, or nine orders of magnitude larger! Any small temperature fluctuation, not to mention pointing the telescope into a hot region, can therefore completely swamp any cosmologically relevant signal, unless properly accounted for.
QUIET data analysis in Norway
And it is at this stage that the University of Oslo and NOTUR enters the story: CMB data analysis is a complicated and computationally very demanding business. In order to detect the extremely valuable 0.1 muK signal, buried deep beneath a sea of instrumental noise, not to mention various systematic effects and astrophysical irrelevant features (such as radiation from our own Milky Way), one has to scan through tens of terabytes of data repeatedly, looking for subtle correlations.
This requires expertise not only in physics and astronomy, but also in statistics, image analysis, and, most importantly, in high-performance computing. And this is one of the main strengths of the Institute of Theoretical Astrophysics compared to the international astrophysics community in general: Through a dedicated and targeted effort during many years, the institute has built up world-leading expertise in high-performance computing for astrophysical problems, both in terms of manpower and hardware. And this is precisely why the University of Oslo was invited into the QUIET experiment in the first place.
However, it is not sufficient only to have the expertise if one does not also have the computer resources to match it. For QUIET, the NOTUR and UiO-based cluster Titan provides the computational power, while NorStore provides the storage space. Without these resources (and the user support provided by the Titan support staff!) it would have impossible to successfully complete the analysis on Norwegian ground. Conversely, with these resources Norwegian scientists can actually play a leading role in one of the biggest games in modern cosmology - the search for primordial gravity waves.
Preliminary results and a look towards the future
On December 23rd 2010, the first phase of the QUIET experiment ended. At that time, about 100 first-generation test detectors had been looking towards the sky for two and a half years, 24 hours per day, 7 days per week, and shutting down only during (occasional) bad weather or maintenance. This phase was designed to test the detectors, rather than making the ultimate B-mode measurement, before asking funding agencies for the necessary funds to build the real experiment.
Fortunately, we are very happy to report that the pilot phase was a great success. Using only the data taken during the first nine months of operations, the first QUIET results were submitted to the Astrophysical Journal on December 14th, 2010, already providing the second best measurement in the world of the CMB polarization signal; only one South Pole based experiment claims slightly lower upper bounds on the gravity wave amplitude. Once the results from the full QUIET data set, comprising two and half years worth of data, is published later this year, it is clear that QUIET will set a new clear world-record in the hunt for primordial gravity waves.
As thrilled as we are with this success, there is of course no time to rest on our laurels. Other experiments are improving their detectors and methods, and so are we. In fact, the QUIET detector development group has already managed to reduce the noise by more than a factor of three relative to the pilot phase detectors, and this improvement alone is equivalent to having ten times as many detectors in the telescope. The plan is to field 500 of these detectors either in 2013 or 2014, and let them scan the sky for up to five years.
If so, QUIET has a real fighting chance to see the warping and twisting of space that happened only 10-34 seconds after the Big Bang, and which eventually created everything we see around us in the universe today. Not bad for
a small telescope sitting at a mountain top in Chile, gazing toward the skies.